CN111131122B - Optical transmission system equalization method and device based on DMT modulation and beat frequency detection - Google Patents
Optical transmission system equalization method and device based on DMT modulation and beat frequency detection Download PDFInfo
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- CN111131122B CN111131122B CN201911411400.0A CN201911411400A CN111131122B CN 111131122 B CN111131122 B CN 111131122B CN 201911411400 A CN201911411400 A CN 201911411400A CN 111131122 B CN111131122 B CN 111131122B
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2626—Arrangements specific to the transmitter only
- H04L27/2627—Modulators
- H04L27/2628—Inverse Fourier transform modulators, e.g. inverse fast Fourier transform [IFFT] or inverse discrete Fourier transform [IDFT] modulators
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/501—Structural aspects
- H04B10/503—Laser transmitters
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/516—Details of coding or modulation
Abstract
The invention discloses an optical transmission system equalization method and device based on DMT modulation and beat frequency detection, which relate to the technical field of optical fiber transmission. And then, aiming at the DMT signal only with the real part, the signal is recovered by adopting a traditional digital signal processing mode, and finally the recovered signal is obtained. The scheme has the main advantages that the imaginary part of the DMT signal is removed, the noise of the DMT signal is reduced, and the performance of the DMT signal is greatly improved under the condition of beat frequency detection.
Description
Technical Field
The invention relates to the technical field of optical fiber transmission, in particular to an optical transmission system equalization method and device based on DMT modulation and beat frequency detection.
Background
The Discrete Multi-Tone (DMT) modulation technique is a variant of the conventional Orthogonal Frequency Division Multiplexing (OFDM) modulation technique, and is widely used in a short-distance optical fiber transmission system because of its flexible subcarrier allocation technique. However, the optical fiber transmission system based on DMT modulation format is affected by the optical fiber dispersion, so the transmission distance is limited to 80 km. In order to extend the transmission distance and make the DMT optical modulation technique suitable for metro network transmission, an optical beat frequency detection method is adopted, and a beat frequency detection method is used to obtain the optical field of the signal, so that the influence caused by optical fiber dispersion can be overcome.
However, for optical DMT signals, performance is affected by laser phase noise during beating. At the same time, the received DMT signal is a complex signal, the imaginary part of which is superimposed with noise. Fig. 1 is a flowchart illustrating a conventional DMT signal digital signal processing recovery process. The received digital signal is first single sideband filtered and then dispersion compensated for complex signals. And performing clock synchronization and frequency offset estimation on the downsampled signal when the downsampled signal is downsampled to 1 time of sampling rate. And then FFT and frequency domain channel compensation and phase compensation are carried out on the signals. And finally, carrying out decision and decoding operation on the recovered signal. Therefore, the conventional DMT signal digital signal processing method increases the noise of the DMT signal, and seriously affects the transmission performance of the optical DMT signal.
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to provide an optical transmission system equalization method and device based on DMT modulation and beat frequency detection, which reduces the noise of DMT signals and improves the recovered telecommunication noise ratio.
In order to achieve the above purposes, the technical scheme adopted by the invention is as follows: an optical transmission system equalization method based on DMT modulation and beat frequency detection, comprising the steps of:
carrying out modulation compensation processing on the DMT signal after beat frequency receiving, and carrying out down-sampling on the processed signal;
carrying out phase pre-compensation on the down-sampled signal;
removing the imaginary part of the DMT signal after the phase pre-compensation, and only keeping the real part of the DMT signal;
and performing signal recovery on the real part of the DMT signal to obtain a recovered signal.
On the basis of the technical scheme, the method for performing phase pre-compensation on the down-sampled signal specifically comprises the following steps:
low-pass filtering the down-sampled signal to filter out the low-frequency part of the signal;
and after normalization and reciprocal processing are carried out on the low-frequency part signals, the low-frequency part signals are multiplied by the down-sampled signals to obtain signals after phase precompensation.
On the basis of the technical scheme, the method for recovering the real part of the DMT signal to obtain the recovered signal specifically comprises the following steps of:
performing FFT, frequency domain channel compensation and phase compensation on the real part of the DMT signal;
and performing decision and decoding operations on the recovered signal.
On the basis of the above technical scheme, the modulation compensation processing is performed on the DMT signal after beat frequency reception, which specifically includes the following steps:
and carrying out single sideband filtering, clock synchronization and frequency offset compensation and dispersion compensation on the DMT signal after beat frequency reception.
The present invention also provides an equalizing apparatus for an optical transmission system based on DMT modulation and beat frequency detection, comprising:
a sample processing module to: carrying out modulation compensation processing on the DMT signal after beat frequency receiving, and carrying out down-sampling on the processed signal;
a phase precompensation module for: performing phase precompensation on the down-sampled signals;
a real part extraction module to: removing the imaginary part of the DMT signal after the phase pre-compensation, and only keeping the real part of the DMT signal;
a signal recovery module to: and performing signal recovery on the real part of the DMT signal to obtain a recovered signal.
On the basis of the above technical solution, the phase precompensation module is specifically configured to:
low-pass filtering the down-sampled signal to filter out the low-frequency part of the signal;
and after normalization and reciprocal processing are carried out on the low-frequency part signals, the low-frequency part signals are multiplied by the down-sampled signals to obtain signals after phase precompensation.
On the basis of the technical scheme, the phase precompensation module comprises a low-pass filter, a normalization unit and a reciprocal unit which are sequentially connected.
On the basis of the technical scheme, the bandwidth of the low-pass filter is twice of the line width of a laser in an optical transmission system for beat frequency detection.
On the basis of the above technical solution, the real part extraction module specifically includes:
an FFT unit for: performing FFT operation on the real part of the DMT signal;
a frequency domain channel compensation unit to: performing frequency domain channel compensation on the signal after the FFT operation;
a phase compensation unit for: performing phase compensation on the signal subjected to frequency domain channel compensation;
a decision decoding unit for: and performing decision and decoding operations on the recovered signal.
On the basis of the above technical solution, the sampling processing module specifically includes:
a single sideband filtering unit to: performing single sideband filtering on the DMT signal after beat frequency receiving;
a clock synchronization and frequency offset compensation unit for: performing clock synchronization and frequency offset compensation on the signal subjected to the single-sideband filtering processing;
a dispersion compensation processing unit for: performing dispersion compensation on clock synchronization and frequency offset compensation;
a downsampling unit to: and carrying out down-sampling on the signal after the dispersion compensation processing.
Compared with the prior art, the invention has the advantages that:
the invention performs channel equalization on the DMT signal after beat frequency reception, is different from the traditional digital signal processing mode, compensates the phase noise of a laser in a frequency domain mode, and removes the imaginary part of the DMT signal, thereby realizing the reduction of noise. And then, aiming at the DMT signal only with the real part, the signal is recovered by adopting a traditional digital signal processing mode, and finally the recovered signal is obtained. The scheme has the main advantages that the imaginary part of the DMT signal is removed, the noise of the DMT signal is reduced, and the performance of the DMT signal is greatly improved under the condition of beat frequency detection.
Drawings
Fig. 1 is a flowchart of a conventional DMT signal digital signal processing recovery process of the background art;
fig. 2 is a schematic diagram of a fiber optic transmission system for beat frequency detection of DMT signals according to an embodiment of the present invention;
figure 3 is a flow chart of the proposed DMT signal digital signal processing recovery according to an embodiment of the present invention;
FIG. 4 is a flow chart of an algorithm for phase pre-compensation used in an embodiment of the present invention;
figure 5 is a graph comparing the performance of DMT signals processed using the method of an embodiment of the invention with the performance of DMT signals processed using a conventional scheme.
Detailed Description
Embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.
The scheme provided by the embodiment of the invention can be applied to any optical DMT signal based on a beat frequency receiving mode. The following describes a beat frequency receiving DMT signal transmission system based on a single photodetector in further detail by taking the example of the present invention as an example.
Fig. 2 is a schematic diagram of an optical fiber transmission system for receiving an optical DMT signal at beat frequency according to an embodiment of the present invention. The transmitting end of the transmission system is provided with two lasers, and the frequency difference between the two lasers is not smaller than the bandwidth of the electric DMT signal. Light emitted by one path of laser passes through the Mach-Zehnder modulator to convert the electric DMT signal into an optical DMT signal. The optical DMT signal is then coupled to another carrier signal. The coupled optical signal enters an optical fiber link after passing through an Erbium-doped fiber amplifier (EDFA). After being transmitted by the optical fiber link, the optical signal is amplified by the EDFA and directly enters the photoelectric detector to be converted into an electric signal. Then, after analog-to-digital conversion, the analog electric signal processes the digital signal so as to finally recover the signal.
Referring to fig. 3, an embodiment of the present invention provides an equalizing method for an optical transmission system based on DMT modulation and beat detection, including the following steps:
carrying out modulation compensation processing on the DMT signal after beat frequency receiving, and carrying out down-sampling on the processed signal;
carrying out phase pre-compensation on the down-sampled signal;
removing the imaginary part of the DMT signal after the phase pre-compensation, and only keeping the real part of the DMT signal;
and performing signal recovery on the real part of the DMT signal to obtain a recovered signal.
Referring to fig. 4, as a preferred embodiment, the phase pre-compensation is performed on the down-sampled signal, which specifically includes the following steps:
low-pass filtering the down-sampled signal to filter out the low-frequency part of the signal; the bandwidth of the low pass filter is equal to twice the laser linewidth.
And after normalization and reciprocal processing are carried out on the low-frequency part signals, the low-frequency part signals are multiplied by the down-sampled signals to obtain signals after phase precompensation.
As a preferred embodiment, performing signal recovery on the real part of the DMT signal to obtain a recovered signal, specifically including the following steps:
performing FFT, frequency domain channel compensation and phase compensation on the real part of the DMT signal;
and performing decision and decoding operation on the recovered signal.
As a preferred embodiment, performing modulation compensation processing on the DMT signal after beat frequency reception specifically includes the following steps:
and carrying out single sideband filtering, clock synchronization and frequency offset compensation and dispersion compensation on the DMT signal after beat frequency reception.
Figure 5 is a comparison of the performance of a DMT signal using the method proposed by an embodiment of the present invention with the performance of a conventional scheme. Wherein the DMT signal uses QPSK modulation format, and the transmission rate is 100Gb/s. In experimental tests, the line width of the laser is 1MHz, and the transmission distance is 100 kilometers. The telecommunication noise ratio of the received signal is calculated, so that the channel equalization method provided by the invention can theoretically improve the recovered telecommunication noise ratio (SNR) by 3dB compared with the traditional method, and is well consistent with a theoretical value.
An embodiment of the present invention further provides an optical transmission system equalization apparatus based on DMT modulation and beat frequency detection, including:
a sample processing module to: carrying out modulation compensation processing on the DMT signal after beat frequency receiving, and carrying out down sampling on the processed signal;
a phase precompensation module for: carrying out phase pre-compensation on the down-sampled signal;
a real part extraction module to: removing the imaginary part of the DMT signal after the phase precompensation, and only keeping the real part of the DMT signal;
a signal recovery module to: and performing signal recovery on the real part of the DMT signal to obtain a recovered signal.
Preferably, the phase pre-compensation module is specifically configured to:
low-pass filtering the down-sampled signal to filter out the low-frequency part of the signal;
and after normalizing and taking the reciprocal of the low-frequency part signal, multiplying the low-frequency part signal by the down-sampled signal to obtain a signal after phase precompensation.
Preferably, the phase precompensation module comprises a low-pass filter, a normalization unit and a reciprocal unit which are connected in sequence.
Preferably, the bandwidth of the low-pass filter is twice the line width of the laser in the optical transmission system for beat frequency detection.
As a preferred embodiment, the real part extraction module specifically includes:
an FFT unit for: performing FFT operation on the real part of the DMT signal;
a frequency domain channel compensation unit to: performing frequency domain channel compensation on the signal after the FFT operation;
a phase compensation unit for: performing phase compensation on the signal subjected to frequency domain channel compensation;
a decision decoding unit for: and performing decision and decoding operation on the recovered signal.
As a preferred embodiment, the sampling processing module specifically includes:
a single sideband filtering unit to: performing single sideband filtering on the DMT signal after beat frequency receiving;
a clock synchronization and frequency offset compensation unit for: performing clock synchronization and frequency offset compensation on the signal subjected to the single-sideband filtering processing;
a dispersion compensation processing unit for: performing dispersion compensation on clock synchronization and frequency offset compensation;
a downsampling unit to: and carrying out down-sampling on the signal after the dispersion compensation processing.
The respective operations in the embodiments of the present invention may be performed by any appropriate means capable of performing the corresponding functions. The means may comprise various hardware and/or software components and/or modules.
It will be apparent to those skilled in the art that various changes and modifications may be made in the present invention without departing from the spirit and scope of the invention. Thus, if such modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include such modifications and variations.
Claims (6)
1. An equalization method for an optical transmission system based on DMT modulation and beat frequency detection, comprising the steps of:
carrying out modulation compensation processing on the DMT signal after beat frequency receiving, and carrying out down sampling on the processed signal;
carrying out phase pre-compensation on the down-sampled signal;
removing the imaginary part of the DMT signal after the phase pre-compensation, and only keeping the real part of the DMT signal;
performing signal recovery on the real part of the DMT signal to obtain a recovered signal;
the phase pre-compensation of the down-sampled signal specifically includes the following steps:
low-pass filtering the down-sampled signal to filter out the low-frequency part of the signal;
after normalization and reciprocal taking processing are carried out on the low-frequency part signals, the low-frequency part signals are multiplied by the down-sampled signals to obtain signals after phase precompensation;
the method for recovering the real part of the DMT signal to obtain the recovered signal specifically comprises the following steps:
performing FFT, frequency domain channel compensation and phase compensation on the real part of the DMT signal;
and performing decision and decoding operations on the recovered signal.
2. The method of claim 1, wherein the performing modulation compensation on the beat frequency received DMT signal comprises:
and carrying out single sideband filtering, clock synchronization and frequency offset compensation and dispersion compensation on the DMT signal after beat frequency reception.
3. An equalizing device for an optical transmission system based on DMT modulation and beat frequency detection, comprising:
a sample processing module to: carrying out modulation compensation processing on the DMT signal after beat frequency receiving, and carrying out down-sampling on the processed signal;
a phase precompensation module for: performing phase precompensation on the down-sampled signals;
a real part extraction module to: removing the imaginary part of the DMT signal after the phase pre-compensation, and only keeping the real part of the DMT signal;
a signal recovery module to: performing signal recovery on the real part of the DMT signal to obtain a recovered signal;
wherein, the phase precompensation module is specifically configured to:
low-pass filtering the down-sampled signal to filter out the low-frequency part of the signal;
after normalizing and taking the reciprocal of the low-frequency part signal, multiplying the low-frequency part signal by the down-sampled signal to obtain a phase pre-compensated signal;
the real part extraction module specifically includes:
an FFT unit for: performing an FFT operation on a real part of the DMT signal;
a frequency domain channel compensation unit to: performing frequency domain channel compensation on the signal after the FFT operation;
a phase compensation unit for: performing phase compensation on the signal subjected to frequency domain channel compensation;
a decision decoding unit for: and performing decision and decoding operations on the recovered signal.
4. The apparatus of claim 3, wherein the phase pre-compensation module comprises a low pass filter, a normalization unit, and an inverse taking unit connected in sequence.
5. The apparatus of claim 4, wherein the low pass filter has a bandwidth twice a laser linewidth in the beat detection optical transmission system.
6. The apparatus of claim 3, wherein the sample processing module specifically comprises:
a single sideband filtering unit to: performing single sideband filtering on the DMT signal after beat frequency receiving;
a clock synchronization and frequency offset compensation unit for: performing clock synchronization and frequency offset compensation on the signal subjected to the single-sideband filtering processing;
a dispersion compensation processing unit for: performing dispersion compensation on clock synchronization and frequency offset compensation;
a downsampling unit to: and carrying out down-sampling on the signal after the dispersion compensation processing.
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